Nineteen patients, 20 to 47 years old with heroin addiction and renal disease, were studied. Patients with pre-existing systemic disease that can cause renal pathology were excluded. The mean duration of the addiction was 3.8 +/- 8 years. Most of the patients had the nephrotic syndrome. Renal biopsy revealed focal histological changes in 10 patients (5 with focal sclerosis) and diffuse changes in 9. Immunofluorescence studies done in 17 patients revealed diffuse deposition of IgG and beta 1C in 11 patients and focal deposits in 6 (mostly IgM). They were followed for a period ranging from 7 to 67 months. Most patients experienced progressive deterioration of renal function except in 4 cases in whom there was cessation of the drug addiction. These 4 displayed a rise in glomerular filtration rate (from 87 +/- 16 to 100 +/- 11 ml/min), marked reduction in proteinuria (from 6.5 +/- 2.6 to 0.4 +/- 0.1 g/24 hr) and an increase in serum albumin (from 3.2 +/- 0.5 to 4.0 +/- 0.1 g/dl). In summary, the fact that the renal abnormalities of our patients could not be explained by other diseases and the marked improvement upon cessation of the addiction favors the notion that heroin addiction "per se" may be the cause of the nephropathy. However, the heroin nephropathy was not manifested by a specific histological or immunofluorescence pattern.